It has been recently proposed that a particulate filter for capturing particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added between the selective reduction catalyst and the particulate filter, thereby attaining reduction of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between a urea water added position and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, a compact exhaust emission control device as shown in FIGS. 1 and 2 has been proposed by the inventor as Japanese patent application No. 2007-29923. In the exhaust emission control device of the prior application shown, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particles in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8 is a selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. A rear end of the particulate filter 5 is connected to a front end of the selective reduction catalyst 6 through an S-shaped communication passage 9 such that the exhaust gas 3 discharged through the rear end of the particulate filter 5 is forwardly curved back into the front end of the adjacent selective reduction catalyst 6.
As shown in FIG. 2 which shows substantial parts in enlarged scale, the communication passage 9 is the S-shaped structure comprising a gas gathering chamber 9A which encircles the rear end of the particulate filter 5 to gather the exhaust gas 3 through collision of the gas just discharged from the rear end of the particulate filter 5 against a wall surface and substantially perpendicular turnabout of the gas, a mixing pipe 9B which extracts forward the gathered exhaust gas 3 from the chamber 9A and which is provided with an injector 10 for urea water addition (urea water addition means) intermediately of the mixing pipe and a gas dispersing chamber 9C which encircles the front end of the selective reduction catalyst 6 so as to disperse the gas 3 guided by the mixing pipe 9B through collision of the gas against a wall surface and substantially perpendicular turnabout of the gas into the front end of the selective reduction catalyst 6.
Arranged in the casing 7 and in front of the particulate filter 5 is an oxidation catalyst 11 for oxidization treatment of unburned fuel in the exhaust gas 3, and arranged in the casing 8 and behind the selective reduction catalyst 6 is an ammonia reducing catalyst 12 for oxidization treatment of surplus ammonia.
With such construction being employed, particulates in the exhaust gas 3 are captured by the particulate filter 5. The urea water is added intermediately of the mixing pipe 9B and downstream of the filter into the exhaust gas 3 by the injector 10 and is pyrolyzed into ammonia and carbon dioxide gas, so that NOx in the exhaust gas 3 is favorably reduced and depurated by the ammonia on the selective reduction catalyst 6. As a result, both the particulates and NOx in the exhaust gas 3 are reduced.
In this case, the exhaust gas 3 discharged through the rear end of the particulate filter 5 is forwardly curved back by the communication passage 9 into the front end of the adjacent selective reduction catalyst 6. As a result, enough reaction time is ensured for production of ammonia from the urea water since a long distance between the urea water added position intermediately of the communication passage 9 and the selective reduction catalyst 6 is ensured and the flow of the exhaust gas 3 becomes turbulent due to the reversed curving to facilitate mixing of the urea water with the exhaust gas 3.
Moreover, the particulate filter 5 and selective reduction catalyst 6 are arranged in parallel with each other and the communication passage 9 is arranged between and along the particulate filter 5 and selective reduction catalyst 6, so that the whole structure becomes compact in size to substantially improve its mountability on a vehicle.
As a prior art literature pertinent to the invention, there exists, for example, the following Patent Literature 1.    [Patent Literature 1] JP 2005-155404A